As stated use penetrating oil and heat.If an impact driver won't fit ,you can strike the Allen socket with a hammer to shock the bolt.Also,a good heat gun will get the bolt surprisingly hot and safer than a torch near your carpet if it is in the car.Countersunk Allen bolts are generally tight and have a smaller wrench size than a comparable socket head Allen.They tend to strip the heads if rusty or corroded,work slowly at it or you will be drilling it out,or breaking the old plate as said by Gus.